1. Field of the Invention
The present invention relates to electrical switches of the push-button-operated type and, more particularly, to a push button power switch with a thermal overload release and snap-action movements in the breaking and making of contact.
2. Description of the Prior Art
From the prior art are known push button power switches which have a thermal overload release, actuated by an overload current which heats up a bimetallic member to displace a latch, thereby breaking the contacts even if the push button remains depressed. Switches of this type are disclosed in the German Pat. Nos. 15 13 570 and 25 32 734 and in the German Offenlegungsschriften (Published Applications) Nos. 26 26 003 and 26 34 808.
Also known from the prior art are push button power switches which produce a snap-action release movement, independently of the push button return movement, in order to minimize arcing across the contacts, as they separate. An advantageous combination of the snap-action release feature with the thermal overload release feature is suggested in the German Pat. No. 25 32 734.
The disclosed switch has a push button with a hollow stem which is guided in a switch housing for straight-line movements. Inside the hollow push button stem is longitudinally guided a push button plunger which is biased upwardly into the stem by a comparatively weak return spring supported by the housing and has a comparatively strong compression spring arranged between its upper end and the push button. The push button plunger carries a transversely extending switching yoke which is connected to the plunger by means of a yoke pin, so that the switching yoke is pivotable in the manner of a balance beam, while moving up and down with the push button plunger.
One extremity of the switching yoke cooperates with a nose of a latch slide which is guided in the housing for transverse movements, the nose thus forming a fulcrum for the switching yoke. The other extremity of the switching yoke bears against a vertically movable switching slide, the movements of which are transmitted to two or more horizontally cantilevered contact blades, to make and brake contact with cooperating stationary contacts.
A downward movement of the push button is thus followed by a downward movement of the push button plunger and its connected switching yoke, following a partial compression of the stronger compression spring. With the nose of the latch slide acting as a fulcrum for the latching extremity of the switching yoke, its switching extremity drives the switching slide downwardly, in opposition to a return spring, to move the switching blades into abutment with the stationary contacts.
Cooperating with the latch slide is a bimetallic member, heated by an in-series electrical resistance. The bimetallic member is so arranged that an excessive current, indicative of an overload condition, heats and deflects the member to such an extent that the nose of the latch slide is withdrawn from underneath the latching extremity of the switching yoke, allowing the latter to tilt and the switching slide to be lifted by its return spring, even with the push button fully depressed. Accordingly, the push button movement cannot be transmitted to the switching slide, for as long as the bimetallic member remains in its overload condition, with the latch slide withdrawn.
The snap-action contact disengagement produced by a withdrawal of the latch slide under overload conditions is also used for the regular contact disengagement, following an upward release of the push button, with the aid of a special cam slide which is guided for transverse movement in the same direction as the latch slide. A drive pin extending from the push button stem cooperates with the cam slide in such a way that a release movement of the push button produces a movement of the cam slide against the latch slide. As the latch slide is pushed back from under the latching extremity of the switching yoke, in opposition to a latch positioning spring, it produces the same snap-action contact disengagement as when the switch is released through thermal overload.
As the push button moves to its fully released position, its drive pin returns the cam slide to a retracted position, thereby allowing the latch slide to return to its latching position. As suggested in the aforementioned German Pat. No. 25 32 734, the cam slide may be so arranged that it produces a detent action in the depressed position of the push button. This detent action requires a second actuation of the push button, in order to shift the cam slide to a position in which the release movement of the push button is possible.
A shortcoming common to these prior art switches is that their contact making movements are not independent in speed from the push button movements, so that it is possible, for example, for the contact blades to establish only partial contact or to rebound, causing arcing in the process and potential damage to the device controlled by the switch.